398 II. S. WASHINGTON — IGNEOUS COMPLEX OP MAGNET COVE 



The presence of the " Ridge" of hornstone forbids the assumption of a 

 form of laccolith such as those figured by Cross and Pirsson, of consider- 

 able regularity, great comparative depth, and with zones of approxi- 

 mately uniform thickness throughout. 



This difficulty, however, disappears to a large extent if we assume that 

 the mass has a depth very small as compared with the breadth — i. e.,of 

 the shape of a thick disk. In a mass of magma of such a form the 

 zones produced by differentiation would be thicker near the edges, while 

 at the top and bottom they would be much thinner.* Such a thinning 

 out of the zones above would, with the presence of the overlying soil, 

 serve to explain the absence of any of the syenitic rocks inside the horn- 

 stone ridge. This, then, may be regarded provisionally as the form and 

 structure of the mass, but any verification of this hypothesis is for the 

 present impossible. 



But the question whether the mass is a true laccolith, as it seems to 

 be, or a stock or other form of intrusive mass, is, after all, of secondary 

 importance. This would not, so far as we know, materially affect the 

 processes of differentiation. The main point which I have tried to bring 

 out, and on which special emphasis is laid, is that the various abyssal 

 igneous rocks are integral parts of one mass, and that they are of contem- 

 poraneous origin and not due to successive intrusions. This view is in- 

 dicated with great probability, if not conclusively, by the facts already 

 given ; but belief in it will be much strengthened by the petrographical 

 and chemical details to be mentioned presently. 



Part II. — Petrology of Magnet Cove 



DESCRIPTION OF THE MAIN IGNEOUS TYPES 



For the present we shall deal only with the main abyssal types, 

 leaving the rocks which form the small dikes, constituting but a minute 

 fraction of the mass, for a later page. 



In Table I are given analyses of the six main rock types, four of them 

 being given by Williams and the other two made by myself. 



The first of these (I) is of the foyaite of Diamond Jo quarry. It is 

 very light gra} 7 ", generally granitic in structure, but occasionally becom- 

 ing trachytoid. It is composed mainly of orthoclase in large tabular 

 crystals, considerable nepheline, some cancrinite (apparently primary, 

 at least in part), and segirine and segirine-augite. Biotite, sodalite, 

 titanite, and magnetite are rare accessories, but amphibole and apatite 

 are not present. 



*('!'. PirsHon : Twentieth Ann. Rept. V. s. Geol. Survey, 1900, pt. iii, p. ">c.4. 



