422 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 



an explanation. Backstrom, however, expresses himself as strongly 

 against the idea of " diffusion," by which we suppose is meant the dif- 

 fusion of the basic oxides toward the outer cooling surfaces. That such 

 diffusion, however, can take place is clearly shown at Square butte, 

 where it has. In any case a diffusion of some kind must take place or 

 the magma would remain homogeneous. We do not see indeed that 

 Backstrom has advanced any reason which would prove that these two 

 ideas, diffusion and liquation, necessarily exclude each other. We do 

 not see in fact why both may not be operative. 



As a matter of fact, the more that the differentiation of igneous rocks 

 is studied the more evident it becomes that no one simple process will 

 explain all cases, but that to produce such results a variety of factors 

 must be included, any one or all of which may operate to produce a given 

 phenomenon. Such, for example, may be pressure, change of tempera- 

 ture, convection currents (which are shown by the " flow structure " and 

 parallel arrangements of phenocrysts on the margins of intruded masses), 

 diffusion of certain oxide molecules toward cooling surfaces, liquation 

 and crystallization. The operation of these on molten silicate magmas 

 is as yet but little understood and much more must be done and learned 

 before any generally satisfactory theory for differentiation can be ad- 

 vanced. 



Whatever may have been the causes at work at Square butte, two 

 things at least are evident, that the basic oxides concentrated tow^ard the 

 outer edges and that the changes which produced this took place very 

 slowly and with extreme regularity, allowing the differentiation to be 

 very complete and thorough. 



SUMMARY. 



Square butte is a laccolite which has been intruded in Cretaceous 

 sandstones. After the intrusion differentiation took place in the liquid 

 mass, the iron, magnesian and lime molecules being greatly concentrated 

 in a broad exterior zone, leaving an inner kernel of material richer in 

 alumina, alkalies, and silica. This crystallized into a sodalite-syenite, 

 while the outer mass formed a basic granular rock composed essentially 

 of augite and orthoclase, to which the name of shonkinite has been 

 given. After solidification the cooling developed a fine platy structure 

 throughout the mass parallel to the form of the laccolitic cover. Since 

 then erosion has removed the cover, laying bare the laccolite and dis- 

 secting it so that its structure is clearl}^ brought out. 



Owing to the erosion and the platy parting the broad marginal zone 

 of shonkinite has been carved into a wide band of singular monoliths 

 which extends around the mountain on its lower slopes. 



