138 I 1 . Ward — Lighthouse Granite near New Haven, Conn. 



cite in them : also by the chloritization of the biotite, and 

 rarely by the presence of calcite. 



Dynamic Action. — The rock shows the effect of some dy- 

 namic force. The evidence for this lies in several facts : (a) The 

 appearance of the quartz under the microscope — not only does 

 it show undulatory and patchy extinction, but it is also cracked 

 and broken so that at times it is well granulated, (b) The 

 feldspars — there has been a slight cracking along the cleavage 

 directions, and some of the plagioclase lamellge are curved. 

 The development of the microcline basket structure seems to 

 be the result of dynamic action, and where the microcline is 

 more plentiful it is usually accompanied by a correspondingly 

 greater breaking of the quartz. 



The general effect in its greatest expression is to produce a 

 gneissic structure. This varies in degree from simple undula- 

 tory and broken quartz, through specimens where groups of 

 broken quartzes and occasional micas have a roughly sinuous 

 parallelism, to those where the material as a whole is distinctly 

 parallel, shown particularly by the bands of granulated quartz 

 with some accompanying mica. However, the force could not 

 have been extreme, — the adjustment of the rock to the strain 

 was accomplished almost entirely by the breaking of the 

 quartz, the feldspars showing relatively few effects. 



Fault- Contact Phase. — The greenish rock already described 

 as being found in a few places near the fault contact, is seen 

 under the microscope to be a breccia. All the minerals of the 

 granite are cracked, fragmented and granulated. The quartz 

 naturally has suffered the most because of its lack of cleavage. 

 But the feldspars have by no means escaped the shattering ; 

 besides the actual breaking apart of the crystals, the plagio- 

 clase lamellae are bent, folded and faulted ; also some micro- 

 cline has been developed. 



As far as mineral content goes, this rock is the same as the 

 main granite with a few modifications, i'. e., there is no biotite,. 

 for it has all been altered to chlorite, which gives the rock its 

 greenish cast ; muscovite is more abundant, — it occurs as dis- 

 seminated flakes, or as specks and threads scattered quite 

 freely in the groundmass and cracks ; a little clay has been 

 developed, both this and the muscovite indicating the presence 

 of heated waters with the dynamic action. 



Figure 2 is a free-hand drawing of a thin section of this rock 

 viewed with crossed nicols. This shows to what extent the 

 crystals of quartz (Q) and feldspar (F) have been broken and 

 disrupted. Fragments of these minerals of all sizes fill in the 

 cracks and interspaces; these with sprinklings of muscovite, 

 chlorite, a small amount of clay, and a very few accessory 

 minerals make up the groundmass. 



