THE UOLYOKE SHEET. 453 



coarse-grained, rusty saudstone below is but slightly indurated, and for 

 ouly a small distance. The diabase is aphanitic and full of steam holes 

 for 1 3 feet up, and contains in great number angular fragments and long 

 filaments of a drab, tine-grained, compact argillaceous limestone, up to 6 

 inches in length, together with fragments of a fine-grained micaceous sand- 

 stone. The two are often kneaded together, as if both had been plastic. 

 The lower foot of the trap is quite free from inclusions, and the pores are 

 here lai"ge, distant, and more like the cavities formed by the upward motion 

 of the steam than by simple expansion. 



Both these rocks ai'e represented in the Chicopee shale, and this point 

 is at the northern limit of this series. They are unlike the coarse saud- 

 stone on which the trap rests; so that it is not impossible that the trap may 

 be slightly faiilted upon the sandstone at this point. 



This is the material which was classified by President Hitchcock as a 

 variety of greenstone, under the title "indurated clay," and the locality 

 given above is the only one cited.' 



PETROGRAPHICAL DESCRIPTION. 



Under the microscope the limestone is fine-grained, with many grains 

 of quartz as well as of calcite. In a narrow, superficial layer, ^""" wide, at 

 the contact of trap and limestone the limestone is recrystallized as a much 

 coarser and purer calcite. Along one portion of this zone the cross sections 

 of distinct, sharp scalenohedra appear, and these are now covered by a layer, 

 i™™ thick, of a finely fibrous mineral. Rarely there occurs in this zone a 

 long blade with rounded end extinguishing longitudinally and inclosing 

 rounded grains of calcite resembling those included in the Laurentian 

 apatites. The mineral seems to be tremolite. The calcite scalenohedi'a 

 rest on the diabase at the contact line, and project into the recrystallized 

 zone of the calcite, where they are surrounded by the colorless fibrous 

 layer (aragonite?), which is of constant thickness, and upon this rests a 

 botrvoidal layer of ankerite or siderite in simple rhombohedra, with rust 

 marking the cleavage, and above this a coarsely crystalline calcite. 



In the above section the diabase is typical and is unchanged up to tlie 

 contact, and the recrystallized band gives no evidence of high temperature. 

 In a second section, cut a few inches from the first, the results are quite 

 difi"erent. There is no zone of coarser crystallization surrounding the 



' Kept. Gcol. Mass., 1835, p. 409; 1841, p. 644. 



