When the stream has been covered for ages the bristling char acter of the fragments is modified ; the surfaces are somewhat smoothed : and in the illustrations to be cited from the older rocks the structure becomes concentric, and the mass resembles an agglomerate.
Judge Hitchcock says further of the aa : "This is always the first lava running from all outbursts on Mauna Loa and Kilauea. Pahoehoe does not run from under aa, but commences near the fountain head as pahoehoe. The liquid lava under aa is aa, not pahoehoe, as seen in i88o-i by Professor Alexander and myself. A stream of liquid aa flowed from under aa rock and cooled into aa." At first it was supposed that the aa was a clinker field; the breaking up of a partially cooled pahoehoe stream, like river ice in a spring freshet. Later it was thus expressed by Captain Dutton, "When these lavas are discharged they come up out of the ground in enormous volumes, are intensely heated, and are very liquid. As they become cooler they become viscous. The cooling takes place upon the surface of the mass while the interior still remains hat and preserves a viscous liquidity. The fields of aa are formed by the flowing of large masses of lava while in a condition approaching that of solidification. The same stream may exhibit pahoehoe or aa according to the circumstances at tending the flow, and the final form which the stream takes is quite independent of the chemical constitution of the lava." Professor Brigham says: "The aa seems to occur when the lava meets with an impediment, which gives way just as the lava is granulated, rolling the spongy mass aver, and building up huge piles from which the liquid drains away." W. L. Green remarks that the aa looks like a great scoriaceous railway embankment "down the center of which the lava con tinues to flow in a molten state, forming ultimately a solid arched crust which falls in from contraction, though the tube may be miles in length. The convexity is the same in all streams, and there is no necessity for invoking the presence of water." Other observers insist that the first lava issuing from the Ha waiian volcanoes is always aa.
E. P. Baker writes: "I have stood by a wholly molten stream of lava which miles below was cooling into aa." Dr. S. E. Bishop follows D. H. Hitchcock's general statements, and claims that the vesicles of pahoehoe are spherical ; but as the fragments roll along half cooled, the vesicles are pulled out of shape and ragged misshapen forms are developed.
Professor Dana concludes : T. That the differences between pahoehoe and aa must be connected with some condition in the region flowed over. 2. That the conditions must be such as to allow extreme liquidity in the one and a pasty state in the other.
3. That some deeply acting cooling agency has acted upon the lava to make aa. 4. The cooling was from below upwards and suggests ; 5, that subterranean moisture may have been the cool ing agent.
To make this suggestion worthy of consideration, it would be necessary first to show from the actual distribution of the aa that moisture was more abundant beneath it than under the pahoehoe. Water appears in streams and pools, or to a limited extent as rainfall ; but does not seem to show any partiality for the one class of the lavas rather than the other. There was an interesting illustration of the behavior of a lava stream in Catania, Sicily, in 1843. A stream of lava had invaded the cultivated land. Sud denly its extremity was seen to swell up like an enormous blister and then to burst, discharging a quantity of steam with a volley of fragments, solid and liquid. Sixty-nine persons were injured. The catastrophe appeared to have been caused by the lava flow ing over a subterranean reservoir of water, thus suddenly gener ating steam that caused the explosion.
Other ways in whch the lava streams have been affected by water have been cited, none of them resembling aa.
The aa is not confined to Hawaii. I have noticed streams of it in California when passing through the Mojave desert. It has been recognized by J. Morgan Clements in the ancient rocks of the Vermilion iron bearing district of He found many bunches of igneous rocks having a concentric struc ture, and refers them to the pseudo-bombs of aa, based upon the descriptions of Professor Dana. Since then I have referred diorite dikes in the Silurian rocks of the Ammonoosuc district of New Hampshire to the same Another reference may be to Newfoundland ; and I recognize the same structure in some of the Triassic traps near Greenfield, Massachusetts. In ex amining sections of the Hawaiian basalts in many localities one can easily recognize beds of aa among the various components. In these cases the rough surfaces have been smoothed down. Bunches may be recognized by their concentricity, and possibly by an irregular vesiculation. They must not be confounded with the spherical or rounded masses analogous to the columnar struc true.