CALCAREOUS FORMATIONS OF THE SOLOMON GROUP. 561 



calcareous rocks imbedded in an earthy calcareous matrix, together with 

 another dark coloured rock of doubtful volcanic origin. It is therefore 

 apparent that this small island of Santa Catalina existed as a patch of coral 

 reef contemporary with the existence of Santa Anna as an atoll ; and that the 

 breccio-conglomerate was formed when the submerged peak, on which the 

 patch of coral reef had based itself, came within the limit of breaker-action. 



The Shortland Islands. 



This small group of islands consists of one principal island, named Alu, 

 with a number of smaller off-lying islands and islets. Alu Island, which has a 

 breadth of eleven or twelve miles and an elevation of about 500 feet, is composed 

 in its N.W. portion of old and originally deep-seated volcanic rocks (mostly 

 quartz diorites) ; while the greater part of it, together with the off-lying lesser 

 islands and islets, may be described as made up of more recent calcareous 

 formations. 



The accompanying section of Alu (PI. CXLIV. fig. 8) will explain the struc- 

 ture of this island and will show the relation to each other of the volcanic rocks, 

 the coral limestones (so called), and the soft Pteropod and Foraminiferous 

 deposit which apparently makes up the mass of the island. The two off-lying 

 islets, shown in this section, are two broken lines of barrier-reefs which skirt the 

 weather or S.E. coast of Alu Island. The innermost (marked B) is an ancient 

 reef which has experienced an elevation of about 200 feet above the sea; while 

 the outer reef (marked C) has been in great part formed at the present sea-level, 

 having in no part been upheaved more than 20 feet above the sea. Both lines 

 of reef have within them a lagoon-channel, which in the case of the inner or 

 ancient line of barrier-reef is shoaling and evidently filling up. 



The Shortland Islands have been upheaved along an extensive line of 

 barrier-reef that skirts the eastern extremity of the adjacent large island of 

 Bougainville at a distance of about 15 miles from the coast ; and a conception 

 may be obtained of the history of their formation by referring to the section 

 before alluded to. Here we have the original land of volcanic rock in the 

 N.W. portion of the group, from which, as from a nucleus, line after line of 

 barrier-reef has been advanced in a south-easterly direction based on a foundation 

 of Pteropod and Foraminiferous muds, and forming ultimately as the upheaving 

 movement continued the large island of Alu, which yet preserves in the ridges 

 of its interior these ancient barrier-reefs now removed far from the coast and 

 elevated some hundreds of feet above the present sea-level. If we may judge 

 from the existence off the weather coast of the double line of barrier-reef, before 

 referred to, the island of Alu is still extending itself in the same direction. 



The Soft Pteropod and Foraminiferous Deposit. — This deposit, which in all 



VOL. XXXII. PART III. 4 Z 



