GEOLOGY OF THE NORTH CREEK QUADRANGLE 2/ 



which have been loosely described as bosses, and so rendered in 

 ideal sections, are doubtless of laccolitic or other stratiform shape ; 

 some, not of the largest dimensions, appear to have a pluglike form, 

 with more or less vertical boundaries." The small stocks or bosses 

 of the North Creek region are certainly of this pluglike or pipelike 

 form as shown by the very character of their eroded cross-sections 

 and also by the vertical contacts with the country rock. Among the 

 many fine contacts, w^hich came under the writer's observation, not 

 a single exception to the rule of vertical or practically vertical con- 

 tacts was noted. 



In most cases the long axes of the stocks lie parallel to the folia- 

 tion of the inclosing rock, though there are some notable exceptions. 

 It would therefore seem that the molten intrusives generally followed 

 the lines of least resistance but, even in these cases, the broad ends 

 of the stocks cut sharply across the foliation bands, sometimes for 

 a distance of several hundred yards. Such a phenomenon is well 

 exhibited at the south end of the large stock just south of Mountain 

 Spring lake where a big quarry has been opened up along the contact. 



The gabbro stocks are not at all uniformly distributed over the 

 area of the quadrangle, the largest number being confined to a nearly 

 north-south belt with a wndth of from 3 to 5 miles and extending 

 through the middle of the quadrangle. This belt roughly corre- 

 sponds to the general strike of the foliation. A secondary belt, 

 about I mile wide and 5 miles long near the middle eastern bound- 

 ary of the sheet, contains a dozen small stocks. With a single small 

 exception the whole western side of the quadrangle is devoid of 

 gabbro masses. In the northeastern portion a few stocks occur, but 

 they may really belong to some other belt not yet mapped. Thus 

 we see that the gabbro intrusions were limited to rather well-defined 

 areas or belts. 



Among these gabbro stocks four types of occurrence are especially 

 noteworthy as follows: (i) The normal, dark, basic gabbro with 

 diabasic texture and usually homogeneous throughout; (2) gabbro 

 chiefly of the normal type but with irregular patches or masses of 

 lighter colored rocks of syenitic or dioritic make-up, these j)atches 

 blending with the normal gabbro; (3) the whole stock made up of 

 lighter colored, more acidic rock; and (4) any one of the above 

 types with blocks or inclusions of the country rock. These four 

 types are all primary variations. Exam])les of the last three types 

 will be given later. 



