332 Scientific Geology. 



assuming an argillaceous or arenaceous character ; and in Leyden it 

 passes into distinct argillaceous slate. 



It will be seen by the map, that the Hoosac mountain range, (by 

 which I mean all the mountainous regiori between the vallies of 

 Berkshire and the Connecticut,) is composed mainly of two wedge 

 shaped patches ; the one of gneiss and the other of mica slate ; the first 

 having its acute angle towards the north, the other to wards the south. 

 And yet, according to the map, which shows the direction of the strata, 

 (Plate XV I.) the strata extend uninterruptedly across both the wedges. 

 And such I believe to be the fact. It is my opinion that the mica slate 

 and gneiss pass laterally into each other; that is, as we go north, the feld- 

 spar decreases in quantity until it disappears ; and of course the rock 

 is mica slate ; and so vice versa. But the lateral passage of one rock 

 into another is extremely difficult to prove ; because, on account of the 

 diluvium spread over the surface, we cannot trace a stratum with cer- 

 tainty for any considerable distance. Accordingly this is a subject 

 rarely touched upon by geological writers. I recollect, indeed, but a 

 single statement of any fact resembling this : Dr. Macculloch speaks 

 of the beds of red sandstone in Sky, as "changing their composition 

 even according to the line of their prolongations :"* But they merely 

 change from red sandstone into gray quartz rock ; which might have 

 resulted from the application of heat. Yet I see no insuperable diffi- 

 culty in supposing that in one part of an extensive deposit, gneiss 

 might have been produced, and mica slate in another ; I mean in 

 the same stratum. Some geologists suppose that these rock were pro- 

 duced by direct crystallization from aqueous solution : and on this 

 hypothesis, I can imagine how one portion of the menstruum might 

 be destitute, or nearly so, of feldspar : while the other part should 

 abound in it; especially if the supposed lake or ocean were shallow. 

 Other geologists suppose these rocks to have been originally deposit- 

 ed in the condition of sediment, and that their crystallization resulted 

 from their subsequent partial fusion by heat. And certainly in con- 

 sistency with this hypothesis, may the change of composition under 

 consideration be explained. But as to the fact of such a change in 

 the present case, I would not wish to be very positive without farther 

 examination. It is, however, certain, that much of the gneiss in the 

 Hoosac range so much resembles mica slate, that Prof. Dewey was 

 led to describe it as such. But as it does pass into distinct gneiss to- 



* Western Islands, Vol. 1. p. 307. 



